1. Field of the Invention
The present invention relates to a supercharged engine, and more particularly to an engine having a supercharger of a type in which the intake air is compressed by the pressure of the exhaust gas before it is introduced into the combustion chamber.
2. Description of Prior Art
Hithertofore, proposals have already been made of an engine supercharger which utilizes the pressure of the exhaust gas produced in the engine exhaust passage for compressing the intake air before the intake air is introduced into the combustion chamber. This type of supercharger is considered as being advantageous over turbosuperchargers because it can provide a higher supercharging effect at low speed engine operation. A supercharger of this type generally includes a rotor having a plurality of mutually separated, axially extending gas passages and a casing supporting the rotor for rotation about an axis of rotation. The casing is provided with exhaust gas inlet and outlet openings and intake gas inlet and outlet openings which are located at opposite axial ends of the rotor. The arrangements are such that the intake air is drawn into the gas passages through the intake gas inlet opening and compressed by the pressure of the exhaust gas introduced into the gas passages through the exhaust gas inlet opening. As the rotor rotates, the gas passages are sequentially opened to the intake gas outlet opening so that the intake gas is forced by the exhaust gas to flow into the intake passage communicating with the intake gas outlet opening. Thereafter, the passages are opened to the exhaust gas outlet opening so that the exhaust gas is allowed to flow into the exhaust passage communicating with the exhaust gas outlet opening. Thus, it is required that the exhaust gas inlet opening and the intake gas outlet opening be located axially opposite to each other with respect to the rotor. An example of such a supercharger is disclosed by Japanese patent publication No. 38-1153. The supercharger disclosed by the Japanese patent publiction is of a type wherein the exhaust gas inlet and outlet openings are located at one axial end of the rotor and the intake gas inlet and outlet openings are located at the other axial end so that the exhaust gas and the intake gas change their flow directions in the gas passages. As disclosed in detail by the Japanese magazine "Nainen-Kikan (Internal Combustion Engines)" Vol. 15, No. 179, 1976 June issue, there is also known a so-called through-flow type wherein the gas inlet and outlet openings are arranged so that the exhaust gas and the intake gas flow axially through the gas passages without changing the flow directions.
The supercharger of the aforementioned type is considered as being particularly suitable for diesel cycle engines, but can of course be used in gasoline engines as well.
In this type of supercharger, the passages in the rotor are scavenged by the air drawn from the intake gas inlet opening and passed through the passages to the exhaust gas outlet opening. It should, however be noted that, where the intake gas inlet opening is subjected to a suction pressure, it is likely that the scavenging cannot effectively be performed with the result that the exhaust gas is retained in the passages until the passages are opened to the intake gas inlet opening. This will cause the residual exhaust gas being mixed with the intake gas possibly causing an unstable combustion of the intake gas. It is therefore advisable to make the flow resistance of the intake passage upstream of the intake gas inlet opening as small as possible. For that purpose, the air cleaner provided upstream of the supercharger may be of a relatively coarse type having a relatively small flow resistance. However, the use of such coarse air cleaner is disadvantageous in that relatively large dust particles may be allowed to pass through the supercharger into the engine.